Added instructions for other users

Added instructions for other users, i.e. users that have installed conda through Anaconda or Miniconda and do not want to install conda-forge.

Author: HannoSpreeuw

doc/install/installing.rst

@@ -135,6 +135,21 @@ You can then run the installer using
 Finally, close your terminal window and open a new one to make the ``conda`` command
 properly available.
 
+Alternatively, if you have already installed Anaconda or Miniconda, make sure that you create your environment
+in the appropriate manner, e.g. for a Python 3.13 environment:
+
+.. code-block:: bash
+
+   conda create --name amuse_p313 python=3.13 --channel conda-forge --override-channels
+
+
+which you activate in the usual way:
+
+.. code-block:: bash 
+
+   conda activate amuse_p313
+
+
 .. _installing-amuse:
 
 Installing AMUSE